1. Field of the Invention
The present invention relates generally to an apparatus for side pumping an optical fiber, and, more particularly, to an apparatus for optically pumping an optical fiber from the side that comprises a grating formed on a multimode fiber.
2. Description of the Prior Art
Fiber amplifiers and fiber lasers require high optical pump levels to be injected within the region of the fiber that includes the active medium that provides the optical gain. Presently, the output power from fiber amplifiers and fiber lasers is limited by the amount of optical power that can be injected into the active medium of the fiber. This limitation stems from the approaches that have been used in the past to couple optical pump power into the fiber.
In the case of single-mode doped fiber, wavelength division multiplexing (WDM) couplers are typically used. This requires that the path of the doped fiber be interrupted to provide for a section of fiber to connect to the WDM coupler. Such an interruption complicates the pumping process.
In the case of dual clad doped fiber, a common approach for injecting optical pump power is end pumping. End pumping provides at most only two input ends through which all the injected optical pump power enters the fiber. This physically constrains the number and type of pump sources that are able to be used to inject the optical power. Such a constraint has led to the development of optical side pumping techniques for dual clad doped fiber in which the entire length of the fiber is made available to couple in a much greater number and type of optical pump sources.
Accordingly, side coupling pump light into a doped fiber has been performed in the past. As previously described, the use of WDM allows pump light having a different wavelength from that which is to be amplified to be coupled into the doped fiber at any point along the fiber provided that an interruption is made for the WDM coupler in the doped fiber. The WDM coupler is designed so that pump light is sent into one of the two input fibers and the light to be amplified is sent into the other input fiber. The interaction length between the two closely coupled fibers is precisely selected to enable the pump light to be completely coupled to the fiber with the light to be amplified while the light to be amplified remains in the same fiber. This is relatively complex.
A second method for side coupling pump light is to cut V-grooves into the cladding of dual clad doped fiber. The V-grooves bounce the side-coupled light into a mode of propagation along the fiber. The cutting of the V-grooves, however, generally weakens the fiber.
A third approach for side pumping is to couple light through a prism into a dual clad doped fiber by precisely locating the prism against an exposed cladding interface. This exact positioning, however, is difficult to achieve.
It also should be recognized that gratings have been employed in doped fibers to Bragg diffract the propagating light either back onto itself or to couple the propagating light out of the fiber. But a grating had not been used to couple optical pump light into a fiber to enable amplification or laser action on light at another wavelength.
What is needed, therefore, is an apparatus for optically pumping an optical fiber from the side which comprises a grating formed on a multimode fiber which diffracts a beam of pump light at an angle that is matched to the angle characterizing a propagating mode and that is relatively simple to construct.